A recent advance in the liquid crystal art has been the utilization of tilted chiral smectic materials in a surface-stabilized device geometry to give very high speed bistable switching not found in devices using nematic liquid crystals. U.S. Pat. No. 4,367,924 (Clark et al.) discloses the use of chiral smectic C or chiral smectic H liquid crystal materials in a device with response times of 1000 to 10,000 times faster than previously attained with other liquid crystalline compounds. Clark et al. describe the use of two ferroelectric, i.e., tilted, chiral smectic, liquid crystal compounds: (+)-p-[n-decyloxybenzylidene]-p'-amino-(2-methylbutyl)cinnamate (DOBAMBC) and (+)-p-[n-hexyloxy-benzylidene]-p'-amino-(2-chloropropyl)cinnamate (HOBACPC). These compounds have a number of shortcomings including chemical instability and UV sensitivity. Their inherent short pitch length and high birefringence severely restrict the design parameters of the display devices and their high transition temperatures require the use of thermal management techniques.
In Ferroelectrics, Vol. 58, p. 3-7, 1984, Keller describes phenyl benzoate ester liquid crystal materials, where one of the alkyl groups attached to a phenyl ring is optically active, i.e., chiral. Some of these materials possess a chiral smectic C mesophase and show ferroelectric behavior. Of the compounds that are ferroelectric liquid crystals, however, most also show an undesired cholesteric mesophase and those that do not show the cholesteric mesophase have very narrow ferroelectric temperature ranges.
U.S. Pat. No. 4,393,231 and No. 4,481,149 (Misaki et al.) describe achiral perfluoroalkyl substituted phenyl benzoate ester liquid crystal compounds. Such achiral compounds cannot exhibit chiral smectic mesophases and cannot be used as ferroelectric liquid crystal materials.
V. V. Titov and co-workers have published a number of papers describing fluorinated liquid crystal materials. Titov et al. postulated that replacement of hydrogen atoms with fluorine atoms might cause a change of intermolecular interaction and consequently mesomorphic properties owing to geometric and electronic factors. Two representative papers, [Molecular Crystals Liquid Crystals, Vol. 67, pp 235-240 (1981) and Vol. 47, pp 1-5 (1978)], describe four partially fluorinated alkoxy and several perfluorinated alkyl and alkoxy substituted liquid crystal materials. None of these compounds are optically active and, thus, cannot possess ferroelectric properties.
U.S. Pat. No. 4,256,656 (Beguin et al.) discloses substituted phenyl benzoate esters where one of the phenyl rings is fluorinated. These compounds are not chiral and therefore cannot exhibit ferroelectric behavior. Also, ring fluorination enhances the formation of the undesired nematic mesophase.
Mahler, Walter, et al., in "Smectic Liquid Crystal from (Perfluorodecyl)decane," Mol. Cryst. Liq. Cryst. Letters, Vol. 2(3-4), 1985, pp 111-119, disclose the smectogenic ability of (perfluorodecyl)decane, F(CF.sub.2).sub.10 (CH.sub.2).sub.10 H. This compound exhibits a smectic B liquid crystal phase, but is not chiral and therefore cannot exhibit ferroelectric behavior.